Recent improvements to RC-line encoded position-sensitive proportional counters

Continuing research on the principles of position encoding with RC lines has advanced the design of position-sensitive proportional counters (PSPCs) to meet the requirements for high count rates (>105 counts/sec) and good spatial resolution (>104 spatial elements) in small-angle scattering experiments with x rays and neutrons. Low-noise preamplifiers were developed with pole-zero cancellation in the feedback circuit and modular linear amplifiers with passive RCL shaping which, compared to previous designs, reduce output saturation at high count rates approx.20 times and shorten the position signal processing time to <1 μsec. Using resistive self-heating and propylene decomposition, the conductance and film thickness of pyrolytic-carbon anodes were increased to match the RC time-constants of line PSPCs with the linear amplifiers, to reduce the effects of electron erosion of the anodes at high count rates, and to improve the PSPC linearity. A large-area PSPC (480 x 780 mm2) for low-energy (<60 keV) photon imaging was developed. The electronic uncertainty in the position measurement of 20-keV photons is <0.5 mm (fwhm) in both dimensions. The technology developed for this PSPC is applicable to the design of large-area detectors (>800 x 800 mm2) for the measurement of small-angle scattering with neutrons. The method of electronic thickness discrimination was applied to change the effective thickness of an area PSPC from 12 to 2 cm whenever the molybdenum target of an x-ray generator was changed to a copper target. This thickness adjustment increased the signal-to-background ratio by a factor of approx.6 for the 8-keV photons from the copper target, while maintaining a >90% detection efficiency